As semiconductor devices continue to be manufactured on smaller scales, the systems used to monitor and control fabrication parameters must provide higher accuracy measurements and operate within tighter error margins. Optical metrology systems are commonly used to measure overlay error between layers disposed upon a substrate, such as a semiconductor wafer. However, the industry is now utilizing scanning electron microscopy (SEM) systems as an alternative to the illumination-based systems in order to achieve higher levels of accuracy on the basis of a high resolution achievable by SEM.
One problem with SEM overlay metrology systems is that edge blurring can occur due to rather large e-beam spot diameter compared to pixel size at high SEM magnifications and e-beam interaction volume. It should be noted, that an interaction volume is of critical importance for SEM image. The information depth (depth from which secondaries are emitted) may reach several hundred of nanometers depending on primary electron energy, target material, pattern, etc. The secondary electron signal is displayed at the electron probe position rather than at the actual secondary electron production position. If the scanning electron probe (primary beam) passes an edge on top the patterned surface (as shown in FIG. 1) a diffusion contrast can be created. Detailed description can be found in classic SEM text books like those by A. Hessler-Wyser or Scanning Electron Microscopy by L. Reimer. This diffusion contrast leads to the edge broadening correlated with the scan direction. Such an edge broadening is a fundamental SEM artifact caused by the fact of sequential pixel by pixel image formation. In one aspect, this disclosure is directed (but not limited to) minimizing the effect of edge broadening/blurring on the overlay measured by SEM apparatus.
This edge blurring may have acute influence on precision, accuracy, tool matching, and other characteristics of the overlay measurement. Frame averaging and sophisticated edge detection algorithms may be used to reduce error from edge blurring; however, there is a need for advances in the art that may help to avoid SEM artifacts resulting from edge blurring (e.g., nuisance signals or imaging error due to electron collection at edges).